Four modes, one law.
Pick a tab below — everything recalculates live as you type, and every result shows the formula and the working behind it.
The highlighted box is the answer — it fills itself in as you type.
In volts — 9 for a battery, 5 for USB, 230 for mains.
In amps — 20 mA is typed as 0.02.
In ohms — 1 kΩ is typed as 1000.
In volts — fill any two of the four boxes; the other two are solved.
In amps — 0.5 A is 500 mA.
In ohms — 240 Ω is 0.24 kΩ.
In watts — e.g. 60 for a bulb, 2000 for a kettle.
What powers the LED — 5 V for USB, 3.3 V for a logic board, 9 V for a battery.
Typical values: red ≈ 1.8–2.0 V, yellow/green ≈ 2.0–2.2 V, blue/white ≈ 3.0–3.4 V.
20 mA is the classic indicator-LED current; 10 mA is often plenty bright.
The voltage you want to divide down — e.g. 9 V from a battery.
Sits between Vin and the output node — 10000 Ω is 10 kΩ.
Sits between the output node and ground — Vout is measured across it.
From two knowns to a full answer.
Four steps, no manual required — the calculator does the arithmetic and shows its working along the way.
Choose a mode
Pick the tab that matches your goal — the V·I·R solver, the power calculator, the LED resistor finder or the voltage divider.
Enter what you know
Type your values — everything recalculates live as you type, so there’s no calculate button to hunt for.
Read the working
The headline answer sits in the results panel, with every related quantity, the formula and the substituted numbers beneath it.
Check, copy, repeat
Impossible inputs are flagged and explained in plain language, one-click examples fill the form, and COPY saves the full working.
Every equation behind the results.
Ohm’s law plus its power cousins — V is volts, I is amps, R is ohms and P is watts.
Voltage
Voltage in volts is current in amps times resistance in ohms — the law itself. It gives the drop across any resistance carrying a known current.
Current
Current is voltage divided by resistance. Double the voltage and the current doubles; double the resistance and it halves.
Resistance
Resistance is voltage divided by current — how you size a resistor to limit a current to a chosen value.
Power
Power in watts is volts times amps, working in both directions: a 60 W bulb on 120 V draws 0.5 A.
Power from current
Power rises with the square of current — twice the current means four times the heat. This is what sets a resistor’s wattage rating.
Power from voltage
The third power form, for when you know the voltage across a resistance but not the current through it.
Frequently asked questions.
Short answers about the law, the units, the LED mode and how the calculator behaves.
What is Ohm’s law?+
Ohm’s law says the current through a conductor is directly proportional to the voltage across it, with resistance as the constant of proportionality: V = I × R. Voltage in volts equals current in amps times resistance in ohms. Published by Georg Ohm in 1827, it’s the backbone of circuit analysis — 9 V across a 100 Ω resistor, for instance, pushes 0.09 A (90 mA) of current.
How do I calculate voltage, current or resistance?+
Rearrange V = I × R for whatever is missing: V = I × R, I = V ÷ R and R = V ÷ I. In the first tab, enter the two values you know and pick what to solve for — the answer appears live with the substitution shown. Example: I = 9 V ÷ 100 Ω = 0.09 A.
What is the Ohm’s law triangle?+
A memory aid. Draw a triangle with V on top and I and R side by side underneath, then cover the quantity you want: cover V and you’re left with I × R; cover I and you see V over R; cover R and you see V over I. It’s the same three formulas arranged so they’re hard to forget.
How do I calculate electrical power?+
Start from P = V × I — watts are volts times amps. If you only know two of V, I and R, use P = I² × R or P = V² ÷ R instead. The Power tab accepts any two of V, I, R and P and computes the other two, whichever pair you have.
What units does the calculator use?+
Plain SI units: volts (V), amps (A), ohms (Ω) and watts (W) — so 20 mA is entered as 0.02 and 4.7 kΩ as 4700. Results are shown twice, as the plain value and with engineering prefixes such as mA, kΩ and MΩ, so 0.02 A also reads 20 mA.
Why does an LED need a series resistor?+
An LED fixes its own voltage (its forward voltage) and would draw destructive current if wired straight to a supply. A resistor soaks up the leftover voltage and sets the current: R = (Vs − Vf) ÷ I. The LED tab goes further — it picks the nearest standard E24 resistor value (rounded up, so the current is never above target) and recommends a wattage rating with a 2× safety margin.
What happens if I enter zero or impossible values?+
Nothing silently wrong — the calculator validates every input and explains the problem: dividing by zero (zero current while solving for resistance), power with zero voltage, or a supply below an LED’s forward voltage each get a plain-language message. Physically, zero resistance is a short circuit and zero current is an open circuit.
Is this calculator free, and is my data sent anywhere?+
It is completely free and runs entirely in your browser — nothing you type is uploaded, logged or sent to a server. No account, no sign-up, no limits.